1. Field of the Invention
The present invention relates to a laser module having a semiconductor laser device.
2. Related Background Art
FIG. 6 is a schematic view showing the configuration of a conventional laser module 2. The laser module 2 has a laser diode (hereinafter, referred to as “LD”) therein. In general, an operating current is provided from an LD driver 14 into an LD chip 11 through a wire 21. In a laser module for optical communications, the operating current is modulated for transmitting data. The LD chip 11 generates laser light 15 through electric-optic conversion of the operating current. The laser light 15 is focused by a lens 20 and emitted from the laser module 2.
Electric-optic conversion takes place in the vicinity of the contact point (junction portion) 11a between the wire 21 and the LD chip 11. Accordingly, the output wavelength of the LD chip 11 is largely influenced by the temperature of the junction portion 11a. For example, the output wavelength deviates by 1 pico-meter in response to the temperature change of 0.01° C.
In order to monitor the temperature of the LD chip 11, a thermistor 16 is placed in the vicinity of the LD chip 11, a thermistor 16 is placed in the vicinity of the LD chip 11. The thermistor 16 and the LD chip 11 are mounted on a chip carrier 18. The thermistor 16 can monitor the temperature of the surface of the LD chip 11, but cannot directly monitor the temperature at the junction portion 11a. Therefore, it is difficult to completely control the output wave length of the LD based on the temperature measured by the thermistor 16.
Further, in an optical network using the conventional laser module, it is difficult to transmit data over a long distance. This is because chirp in the signal light varies while the identical bits are output continuously.
FIG. 7 shows the result of measuring chirp regarding the conventional laser module. The line represented by “Power” indicates the output power of the LD, and the line represented by “Chirp” indicates the amount of chirp in the output light from the LD. While the power is at a high level, a bit with a value of “1” is output. On the other hand, while the power is at a low level, a bit with a value of “0” is output. The chirp shown in FIG. 7 is a numeric value obtained by converting the amount of wavelength chirp into a frequency.
As shown in FIG. 7, while bits “1” are output continuously, the chirp gradually decreases. When the chirp varies while a series of bits “1” is output, the wavelength of the signal light is shifted, for example, between the first bit “1” and the last bit “1”. This wavelength shift causes waveform distortion of the signal light due to wavelength dispersion in an optical communication line. The distortion becomes more significant with increasing the transmission distance. Accordingly, it is difficult to transmit data including a series of identical bits over a long distance. The identical bits are likely to continue for a long bit length when the number of the stages of a pseudo random bit sequence (PRBS) is large.